1. Field of the Invention
The present invention relates to golf balls. More particularly, the present invention relates to improvement of cross-sectional shape of dimples.
2. Description of the Related Art
Golf balls have numerous dimples on the surface thereof. A role of the dimples involves causing turbulent flow separation through disrupting the air flow around the golf ball during the flight. By causing the turbulent flow separation, a separating point of air from the golf ball shifts backwards leading to the reduction of a drag coefficient (Cd). The turbulent flow separation promotes the differentia, which result from the backspin, between the separating points at the upper and lower sides of the golf ball, thereby enhancing the lift force that acts upon the golf ball. Flight distance of the golf ball is prolonged on behalf of the reduced drag and enhanced lift force. Aerodynamically excellent dimples promote the turbulent flow separation. In other words, aerodynamically excellent dimples can disturb the air flow more efficiently.
A variety of improvements of the cross-sectional shape of the dimples in an attempt to improve the flight performance have been proposed. U.S. Pat. No. 5,735,757 discloses a golf ball provided with double radius dimples having a predetermined shape. U.S. Pat. No. 6,346,053 discloses a golf ball in which an edge angle and a radius of curvature are set to be within a predetermined range.
Density of dimples (may be also referred to as “surface area occupation ratio”) is included in specifications that may exert a great influence on flight performances of a golf ball. Golf balls having a great density are excellent in flight performance. A variety of proposals have been made in connection with the density. U.S. Pat. No. 4,813,677 discloses a golf ball provided with dimples that are densely arranged such that any new dimple having an area that is greater than the average area can not be formed.
Diameter of the dimple also exerts a great influence on flight performances of a golf ball. Golf balls that exhibit the drag reduced at the initial stage of a trajectory through the formation of numerous dimples having a great diameter have been placed on the market.
At the impact between a golf ball and a golf club, the surface of the golf ball is scuffed by the face line of the golf club. The nap is thereby raised on the surface of the golf ball. Thus resulting nap markedly deteriorates the appearance of the golf ball. Power is liable to converge in the vicinity of the edge of a dimple, where the nap is readily raised. The nap shall remain along the contour of the dimple.
According to golf balls having a great surface area occupation ratio Y of the dimples, substantial contact area at impact shall be small, therefore, great pressure is applied in the vicinity of the edge. Golf balls having a great surface area occupation ratio Y are liable to fuzz up the nap. Because the nap remains along the contour of the dimple as described above, fuzzing up of the nap is remarkable for dimples having a long contour length, in other words, for dimples having a great diameter. There is an urgent need to suppress deterioration of the appearance of golf balls having dimples with a great diameter and having a great surface area occupation ratio Y.
An object of the present invention is to provide a golf ball that is excellent in a flight performance and is resistant to fuzzing up of the nap.